1. Field of the Invention
The present invention relates to gain control techniques for images in an image capture apparatus, such as a camera, that is capable of capturing still images or moving images.
2. Description of the Related Art
Japanese Laid-Open Patent Application No. 63-279124 discloses a light metering apparatus (exposure control apparatus) for use with an image capture apparatus such as a digital camera. In the light metering apparatus, an image capture plane is divided into a plurality of areas. The light metering apparatus obtains luminance information on every area and calculates a correct exposure value based on the obtained luminance information using a predetermined algorithm. Then, on the basis of the calculated exposure value, the image capture apparatus controls a gain to be applied to a captured image signal.
Japanese Laid-Open Patent Application No. 2000-253409 discloses an image capture apparatus, such as a digital camera, having an apparatus capable of providing an appropriate gain control. The image capture apparatus measures reflected light from an imaging plane so as to detect whether an external flash unit suitable for camera studios has been used for flash emission and, on the basis of a result of the detection, determines whether the external flash unit has been used. Then, taking a result of the determination into consideration, the image capture apparatus controls a gain to be applied to an image signal using a predetermined algorithm so as to perform adequate white balance correction.
In general, in single-lens reflex cameras, a light metering sensor is incorporated into a viewfinder optical system having a pentagonal prism. The light metering sensor is often configured to receive part of light rays diffused by a diffusion plate known as a focusing screen. In such a configuration, the light metering sensor is located at a position away from an optical axis of the viewfinder optical system, which is directed to an eyepiece lens. Accordingly, the amount of light incident on the light metering sensor varies in various ways depending on the speed (f-number) of an interchangeable lens, the position of an exit pupil thereof, or the diffusion characteristic of a focusing screen. Therefore, in cases where luminance information is to be obtained from a plurality of areas into which an image capture plane is divided, there is a possibility that the balance of the amount of light between a central portion of the image capture plane and a peripheral portion thereof may change. Accordingly, it may be difficult for the camera to determine whether a scene to be photographed is in a backlight condition or the like, and the actual exposure value may deviate from a correct exposure value. Correcting an output signal from the light metering sensor may be useful for preventing such problems. However, it is practically difficult to perform accurate correction for every interchangeable lens.
In cases where a large flash unit that is usually installed in a camera studio is used, only a modeling light is usually used to illuminate an object before starting exposure. The modeling light is an auxiliary light source that allows the photographer to preview the effects of flash emission. In this condition, when the photographer presses the shutter release button, the digital camera sends to the large flash unit a synchronization signal indicative of the duration in which the shutter is opened. The large flash unit emits flash light in synchronization with the synchronization signal, thus enabling an exposure operation to be performed. In the case of such an exposure operation, it is difficult for the digital camera to determine whether the large flash unit has really emitted flash light, on the basis of only the fact that the digital camera has sent the synchronization signal. Therefore, the digital camera uses, as auxiliary information for an automatic white balance correction method, information indicating that the object and the surrounding area are dark. The digital camera is uncertain whether flash emission has really taken place. Accordingly, the digital camera has to consider a wide range of color temperature as illumination light. Thus, it is often difficult to appropriately perform white balance adjustment depending on the color of an object.